1. Field of the Invention
The present invention relates to alignment control, and in particular to an alignment system that uses an alignment target that includes at two locations having designed in offsets.
2. Discussion of the Related Art
Semiconductor processing for forming integrated circuits requires a series of processing steps. These processing steps include the deposition and patterning of material layers such as insulating layers, polysilicon layers, and metal layers. The material layers are typically patterned using a photoresist layer that is patterned over the material layer using a photomask or reticle. As the integrated circuit feature sizes continue to decrease to provide increasing circuit density, it becomes increasingly difficult to align one layer with another.
During processing, the substrate is moved from one location to the next so that different areas, e.g., dies, on the substrate can be exposed. The alignment system, e.g., the exposure tool, typically uses an alignment target to properly align the substrate during exposure. FIG. 1 shows a conventional alignment system 50, which includes a diffraction grating 52 on the substrate and a second diffraction grating 54 that is stationary, e.g., is fixed to the lens on the exposure tool. A light source 56 produces coherent light that passes through a beam splitter 58 and is incident on the diffraction grating 52 after passing through a lens 60. The light is diffracted by diffraction grating 52 and passes through lens 60 back to beam splitter 58. The coherent light beam from source 56 is also reflected off beam splitter 58, passes through lens 62 and is incident on diffraction grating 54. The light diffracted by diffraction grating 54 passes back through lens 62 to beam splitter 58. At beam splitter 58 the light diffracted from diffraction gratings 52 and 54 is combined and the combined diffraction patterns is received by light detectors 64.
Alignment system 50 provides an accuracy of approximately 15 nm. One disadvantage of alignment system 50 is that coherent light is used. Thus, if the diffraction grating 52 on the sample absorbs the particular frequency used, alignment system 50 cannot provide an accurate measurement. While multiple coherent light sources may be used to avoid this disadvantage, the use of multiple light sources adds complexity and cost.
Thus, there is a need in the semiconductor industry for an improved alignment system.